The screenshot below shows the volume of unique visitors for eVar1 and the page dimension for the same page name.
We have one s.t() call on the page sending the page name to both s.pageName and eVar1. There are a couple of click events on the page, but these fire s.tl() calls so should not inflate the metrics like visits, unique visitors, page views etc. for eVar1.
As you can see, eVar1 unique visitors are generally much higher than unique visitors to the page dimension, until a point in early February when they match up (which should happen anyway).
We only started seeing this mismatch when we migrated to Launch so when things aligned again in February it was a pleasant surprise.
This appears to be a blip because since the last week of February, the unique visitors for the eVar are outstripping the unique visitors for the page once again.
I have to stress, there have been no configuration changes in Launch affecting the page in question. No new events, no changes to existing rules etc. Nothing. There have obviously been changes to our overall Launch configuration (i.e. rules modified or added to other parts of the site), and dev releases to our site, but nothing that would directly affect the Launch code or have an impact on how Analytics itself processes s.t() or s.tl() calls and calculates metrics.
This issue does not affect all pages on our site, and it seems like the only commonality between affected pages is that there are events triggered on the page itself.
I appreciate there is a lot to unpick here and more information will probably be required, but this has been bugging me for months.
Note that our site isn't accessible outside the UK if you want to do some QA.
Just thinking about the real-world scenarios with proper implementation.
Page Views: This should ideally match because Page Name is mandatory to set Page Views.
Visits: How about this scenario? I landed the page (eVar1 and Page Name got 1 visit) and after 30 minutes, I have clicked on a button where event tracking is there. Now Visit will be incremented to eVar1 but not to Page Name because we cannot set page Name in s.tl() call.
Unique Visitors: How about this scenario? I landed the page on 28 Feb 2021 at 11:55 AM and clicked on the event on 01 March 2021 at 12:01 AM. Now Unique Visitor will be counted only to eVar1 but not to Page Name if we select March month alone.
In your case, the numbers are high with a consistent difference so not sure the above Unique Visitor scenario will be valid (Considering day numbers rather than the month as said in the example). I am thinking about the other real-world cases and whenever the Unique Visitors gap arises, mostly the problem would be in Safari because of Cookie Blocking. Give me some time, will get back to you.
If try to compare "evar1 instances" and "pagename instances, or pageviews", will that match?
IMO, evar will be always higher than or equals to prop unless it's expiration set to "hit".